My guess would be that because no one uses anything short of "LGBT" as their choice pride acronym anymore; I swear, half the people that say "LGBT" aloud don't know what even all four letters stand for ("Lesbian, Gay,... bacon, tomato?"). They probably just didn't want to get criticised, but also remember that at least a few guys pretty far up the chain, if I remember correctly, have stated that anti-trans laws for the US military is silly. So if this is more of a locally established dinner party, they might be genuinely supportive of the T in their forces while the higher-ups aren't so much.